Tales of ghosts, wraiths, and other apparitions have been reported in virtually all cultures. The strange sensation that somebody is nearby when no one is actually present and cannot be seen is a fascinating feat of the human mind, and this apparition is often covered in the literature of divinity, occultism, and fiction.
Descending with his brother from the summit of Nanga Parbat, one of the ten highest mountains in the world, Reinhold Messner felt a third climber “descending with us, keeping a regular distance, a little to my right and a few steps away from me, just outside my field of vision.” Messner “could not see the figure” but “was certain there was someone there,” sensing “his presence.” This apparition, the sensation that somebody is nearby when no one is actually present, is called the feeling of a presence, hereinafter “FoP,” and has been described during periods of physical or mental exhaustion and has influenced occult literature and fiction. Although people do not see the “presence,” they may describe its spatial location and frequently turn around or offer food to the invisible presence.
This condition is very different from other bodily hallucinations such as the autoscopic phenomena, which are defined as illusory own-body perceptions, during which patients experience the visual illusory reduplication of their own body in extrapersonal space. Three main forms of autoscopic phenomena have been defined: during autoscopic hallucinations, a second own body is seen without any changes in bodily self-consciousness; during out-of-body experiences (“OBE”), the second own body is seen from an elevated perspective and location associated with disembodiment; finally, during heautoscopy, subjects report strong self-identification with the second own body, often associated with the experience of existing at and perceiving the world from two places at the same time. All these conditions have been linked either to a single and hemisphere-specific lesion brain site or to disorders of multisensory integration that do not involve the sensorimotor system.
Recent developments in video, virtual reality and robotics technologies have allowed researchers to investigate the central mechanisms of bodily self-consciousness by providing subjects with ambiguous multisensory information about the location and appearance of their own body. This has made it possible to study important aspects of bodily self-consciousness, how they relate to the processing of bodily signals and which functional and neural mechanisms they may share. Using robotic technology, it has been possible to achieve specific bodily conflicts and induce predictable changes in fundamental aspects of self-consciousness by altering healthy subjects' bodily self-perception.
Although the FoP has been described in psychiatric and neurological patients, as well as in healthy individuals in different situations, its neural origin is unknown. Abnormal integration of sensorimotor signals and their cortical representations has been described in schizophrenic patients and has been associated with positive hallucinatory and delusional symptoms. According to this view, positive schizophrenic symptoms, such as alien voices and delusions of control, are caused by central deficits in integrating predicted sensory consequences of own movements and the respective reafferent signals. As a consequence, it is hypothesized that schizophrenic patients, under certain conditions, may not perceive self-generated sounds and movements as such but may misperceive them as being generated by external agents (as in the experience of alien voices or control of own movements by others), and this is corroborated by behavioral and neuroimaging investigations.
A single case report showed that electrical stimulation in temporoparietal brain cortex induces the FoP, suggesting that disturbed sensorimotor processing (tactile, proprioceptive, and motor cues) is important. However, this has not been confirmed in other patients, and the significance of these findings for the FoP in healthy subjects is unclear. There is therefore a need in the for a device, system, and method that uses the mechanisms underlying the spontaneous triggering of the FoP, or for the voluntary induction thereof, which could impact not only the basic cognitive neuroscience research and the linked disordered mental statuses, for example but not limited to schizophrenia, psychosis in major depression, depersonalization, but also other domains such as those of videogames and virtual or augmented reality.